THE DEVELOPMENTAL ROLE OF TYPE-III AND TYPE-IV CELLS IN THE RAT SUBMANDIBULAR-GLAND

In addition to seromucous cells and a few mucous cells (Type II cells) , submandibular glands of 2-8 month old rats contain two additional ce ll types: type III and type IV. Type III cells contain serous-type sec retory granules that sometimes have a complex substructure; type IV ce lls appear to be seromucous, but their granules clearly are different from those in conventional endpiece seromucous cells. Both type III an d IV cells are involved in histogenesis of new endpieces in a process that differs markedly from that occurring in perinatal glands. In this process, intercalated ducts bud and give rise to immature endpieces t hat consist entirely of type III cells. These differentiate into type IV cells, which in turn differentiate into standard seromucous cells. Concurrently, the intercalated ducts become shorter as their most dist al cells differentiate into granular duct cells. This type of developm ental process begins similar to 2 months postnatally, when histogenesi s of endpieces by means of terminal tubules has ended, and continues u ntil 6 months, when its frequency sharply declines.